This invention concerns photochromic glasses. More particularly, it concerns a method of controlling and adjusting a permanent surface coloration, developed by heat treatment in the presence of a reducing agent, in a photochromic glass article containing a silver halide as a photochromic agent. Another aspect is the article thus produced.
Characteristically, photochromic glasses darken upon exposure to actinic radiation, particularly ultraviolet light, and fade or lighten upon removal of the activating radiation. Thus far, commercial applications for these glasses have been principally in the fields of ophthalmic and sunglass lenses. The commercially significant glasses employ one or more selected silver halide(s) as their active ingredient, and were first described by Armistead et al. in U.S. Pat. No. 3,208,860.
Since that time, silver halide-containing, photochromic glasses that exhibit improved darkening and fading characteristics have been developed. One such family of glasses is described by G. B. Hares et al. in U.S. Pat. No. 4,190,451.
Photochromic glasses are also known that possess non-neutral coloration by transmitted light in the undarkened state. Glass bodies, in which such coloration is developed throughout the body, may be produced by the addition of known light-absorbing colorants in the initial glass melt.
Alternatively, photochromic glasses may have permanent coloration imparted in a surface layer by a reduction heat treatment. U.S. Pat. Nos. 3,892,582 and 3,920,463 (Simms) disclose such a procedure. The patents teach that pink, yellow and brown colors may be developed by firing, in a reducing atmosphere, an article formed from certain silver halide-containing photochromic glasses.
Photochromic glasses, having a yellow color imparted by the procedure described in the Simms' patents, have been produced commercially. The surface color development is attributed to an absorption band caused by the precipitation of metallic silver in the glass during heat treatment. In silver-containing glasses free of other precipitated phases, the silver absorption band is manifested as an absorption peak centered at about 400-410 nm in the violet region of the spectrum. In the reduction-fired, photochromic glasses reported in the aforementioned Simms patents, which contain precipitated silver halide in addition to silver in the matrix glass, absorption peaks are reported in the blue region of the spectrum between about 430-460 nm.
The hue and intensity of the induced color in prior art glasses probably depended upon the position and intensity of the treatment-induced absorption peak. The deepest yellow colors were caused by strong absorption peaks at 430 to 460 nm, while the light pink color is now thought to have been caused by the same fundamental absorption peak as it first appeared in weak form at about 500 nm following a mild heat treatment.
U.S. Pat. No. 4,240,836 (Borrelli et al.) discloses surface-colored photochromic glass articles exhibiting a wide range of colors in the undarkened state. The colors range from blue to orange, red and purple, and are the result of inducing absorption peaks at longer wave lengths above 460 nm, and frequently in the 510-580 nm range. A primary condition in the Borrelli et al. procedure is restricting the temperature of the heat treatment in a reducing atmosphere to not over 450.degree. C. This minimizes possible melting of the photochromic phase (silver halide) during the reduction step.